SLF 47/Inf.5 - SNAME.org

INTERNATIONAL MARITIME ORGANIZATION
E
IMO
SUB-COMMITTEE ON STABILITY AND
LOAD LINES AND ON FISHING VESSELS
SAFETY
47th session
Agenda item 6
SLF 47/INF.5
10 June 2004
ENGLISH ONLY
REVIEW OF THE INTACT STABILITY CODE
Recordings of head sea parametric rolling on a PCTC
Submitted by Sweden
Executive summary:
Action to be taken: Paragraph 2
Related documents: SLF 47/6/6
SUMMARY
This document provides the text of the report “Recordings of head sea
parametric rolling on a PCTC” referred to in document SLF 47/6/6.
1 Attached at annex is the report “Recordings of head sea parametric rolling on a PCTC”
referred to in document SLF 47/6/6.
Action requested of the Sub-Committee
2 The Sub-Committee is invited to take note of the information in the attached annex when
considering document SLF 47/6/6.
***
I:\SLF\47\INF-5.DOC
For reasons of economy, this document is printed in a limited number. Delegates are
kindly asked to bring their copies to meetings and not to request additional copies.

Recordings of head-sea parametric rolling on a PCTC 1(9)
Recordings of head-sea parametric rolling on a PCTC
Mikael Palmquist, Seaware AB, e-mail: mikael.palmquist@seaware.se
Christer Nygren, Wallenius Marine AB
Summary:
This report presents data that was recorded on the PCTC M/V Aida between February 1 and 4, 2004.
On this occasion, the ship encountered head sea with significant wave height 5-6 m and travelled with
reduced speed 8-10 knots. At five different occasions parametric rolling evolved, with roll angles up to
17 degrees. The strong pitch-roll coupling during head-sea parametric rolling is clearly demonstrated
by recorded motion time series. When relating these recordings to another incident of heavy rolling (up
to 50 degrees read off inclinometer) in head sea in February 2003 on Aida, it is found that the
conditions seem to have been very similar, except for that the significant wave height was larger at that
time.
The recordings were made by an on-board operational decision support system. This system
implements MSC/Circ.707 as a basis for operational support regarding dynamic stability phenomena
such as parametric rolling. When running the system in playback mode with modified settings, i.e.
reduced wave height threshold and including also head sea, warnings for parametric rolling are issued
in route planning mode two days ahead as well as in automatic mode 10 hours prior to the first
occurrence.
Finally, revision of MSC/Circ.707 and the importance of operational guidance are briefly discussed.
1 Introduction
In February 2003, the Wallenius PCTC M/V Aida experienced sudden violent rolling in rough head sea
southwest of the Azores. Roll angles as large as 50 degrees were read off the bridge inclinometer.
When this incident was post-analysed it was found that the conditions, in terms of the relation of wave
encounter period and natural roll period, were such that parametric rolling was the most likely cause.
Partly due to this incident, M/V Aida was equipped with a Seaware EnRoute Live ® system in
December 2003 for trial during the winter.
The EnRoute system is an on-board based operational decision support system. It includes both voyage
planning based on weather forecasts and real-time numerical ship dynamic simulations, 6 d.o.f. motion
measurements, on site wave spectrum evaluation and guidance to the master in dangerous situations.
Examples of dynamic effects for which the system can provide warnings and guidance are slamming,
green water on deck, excessive cargo lashing forces and dynamic stability. In addition to this it
provides a recording functionality that continuously records parameters such as ship motions, wind,
ship speed and heading, evaluated seastate and more. Thanks to the recording functionality, parametric
rolling has now, to the authors’ knowledge for the first time ever, been recorded during normal ship
service.
This report presents data that was recorded on M/V Aida in heavy weather conditions between
February 1 and 4, 2004, on a voyage from Southampton to New York. On this occasion, Aida
encountered heading sea with significant wave height 5-6 m and travelled with reduced speed 8-10
knots west of the Azores. At five different occasions parametric rolling evolved. Luckily, the rolling
did not reach the extreme amplitudes of last year’s incident.
2 Recorded data
2.1 Voyage overview
Figure 1 below shows recorded data during the entire voyage. The first graph shows measured heave,
pitch and roll in terms of significant values during 2 minutes blocks. As seen, sudden and relatively
large rolling occurred 5 times during 2 and 3 Feb. The second and third graphs show mean values of
wind speed and relative wind direction during 2 minutes blocks. Relative wave direction is defined so
that 0 degrees represents head wind (positive values means wind on starboard side, negative on port
side). The fourth (lower) graph displays significant wave height and mean zero-crossing period as
estimated by the EnRoute system.

Recordings of head-sea parametric rolling on a PCTC 2(9)
18
16
14
12
10
8
6
4
2
0
Heave (m)
Pitch (deg)
Roll (deg)
40
30
Wind speed (m/s)
20
10
0
150
10 0
50
0
-50
-100
-150
Relative wind direction (deg)
12
10
8
6
4
2
0
01-30
00:00
01-31
00:00
02-01
00:00
Significant wave height (m)
Mean zero-crossing period (s)
02-02
00:00
02-03
00:00
02-04
00:00
02-05
00:00
02-06
00:00
02-07
00:00
02-08
00:00
02-09
00:00
Ti m e ( M M - D D H H :MM )
Figure 1. Overview of the voyage in terms of recorded motions, wind and wave conditions as estimated
by Seaware EnRoute Live.
2.2 Recorded roll motion time series
Figures 2-6 show the roll motion time series of the 5 events of large rolling identified from the voyage
overview. In all of these events, the roll amplitude increases quite rapidly, followed by a decrease back
to the original amplitude level. The fact that the roll motion suddenly increases from about 2-3 degrees
up to as much as 17 degrees in a few cycles (e.g. figure 6) in heading waves clearly indicates the
presence of a non-linear phenomenon.
Roll (deg)
Measured roll. Reference time (t=0) is 2004-02-02 12:41:24UTC
8
6
4
2
0
-2
-4
-6
-8
-10
-12
0 100 200 300 400Time (s) 500 600 700 800 900
Figure 2. Measured roll motion 2004-02-02, reference time 12:41 UTC

Recordings of head-sea parametric rolling on a PCTC 3(9)
Roll (deg)
Measured roll. Reference time (t=0) is 2004-02-02 14:49:24UTC
20
15
10
5
0
-5
-10
-15
-20
0 100 200 300 400Time (s) 500 600 700 800 900
Figure 3. Measured roll motion 2004-02-02, reference time 14:49 UTC
15
Measured roll. Reference time (t=0) is 2004-02-03 06:41:22UTC
Roll (deg)
10
5
0
-5
-10
-15
0 100 200 300 400 Time 500(s)
600 700 800 900 1000
Figure 4. Measured roll motion 2004-02-03, reference time 06:41 UTC
15
Measured roll. Reference time (t=0) is 2004-02-03 15:17:39UTC
Roll (deg)
10
5
0
-5
-10
-15
0 100 200 300 400Time (s) 500 600 700 800 900
Figure 5. Measured roll motion 2004-02-03, reference time 15:17 UTC
Roll (deg)
Measured roll. Reference time (t=0) is 2004-02-03 15:58:11UTC
20
15
10
5
0
-5
-10
-15
-20
0 100 200 300 400Time (s) 500 600 700 800 900
Figure 6. Measured roll motion 2004-02-03, reference time 15:58 UTC

Recordings of head-sea parametric rolling on a PCTC 4(9)
2.3 Roll and pitch coupling
Analysis of the correlation between roll and pitch motion during the events of large roll angles provides
very clear examples of parametric rolling. As seen in figure 7 and 8, the pitch period is half the period
of roll during the roll build-up, and is in-phase in such a way that roll extremes coincides with
maximum bow down pitch combined with minimum heave. It is also interesting to note that rolling
starts to diminish when the pitch amplitude decreases or becomes phase shifted relative to the rolling.
In order to archive a better apprehension of the nature of parametric rolling, an animation was created
in which a 3-D model of the ship moves according to the measured motions (2004-02-03 at 16 UTC,
figure 6). A wave surface was added to the animation scene, consisting of regular waves whose wave
period equals to the peak period of the estimated wave spectrum at the corresponding time. The
phasing between the regular wave and the ship was set so that it corresponded to parametric rolling
conditions (wave crest midship at zero roll, wave trough midship at maximum absolute roll angle). The
images of figure 9 are taken from this animation.
It is well known that parametric rolling in head seas involves a strong pitch-roll coupling. Due to this
coupling, the relation between the natural periods of pitch and roll is an interesting factor. In case the
natural pitch period is half of the natural roll period (and therefore equal to the encounter wave period
in parametric rolling conditions), parametric rolling is further encouraged. It should be noted that this
coupling is less important in following waves. On this voyage the loading condition was such that the
natural pitch period actually was close to half the natural roll period (table 1).
The possibility of predicting natural periods of pitch and roll in operational decision support systems
enables early warnings in the route planning stage using weather forecasts with proper wave data.
Table 1 compares predicted (as calculated when leaving Southampton) and measured natural periods of
pitch and roll.
Table 1. Comparison of predicted and measured natural periods.
Predicted by Seaware EnRoute Live From measurements
Natural roll period 23.3 s 22.5 – 24 s
Natural pitch period 11.4 s 10 – 11 s
Measured motion time series. Reference time (t=0) is 2004-02-02 14:55:48UTC
15
10
Heave (m)
Pitch (deg)
Roll (deg)
5
Motions
0
-5
-10
-15
40 60 80 100 120 140 160 180 200 220
Time (s)
Figure 7. Close-up of time series in figure 3, including also heave and pitch.

Recordings of head-sea parametric rolling on a PCTC 5(9)
20
15
10
Measured motion time series. Reference time (t=0) is 2004-02-03 16:06:44UTC
Heave (m)
Pitch (deg)
Roll (deg)
5
Motions
0
-5
-10
-15
-20
0 20 40 60 80 100 120
Time (s)
Figure 8. Close-up of time series in figure 6, including also heave and pitch.
Figure 9. Snapshots from animation of the time series in figure 6 and 8. The maximum pitch (bow
down) coincides with roll extremes and minimum heave.
2.4 Wave conditions
The wave spectra evaluated by the EnRoute system at the times of parametric rolling has peak periods
that, when transformed to encounter periods with respect to ship speed and wave direction, are very
close to parametric resonance conditions, i.e. half the natural roll period. In order to confirm these
spectra, hindcast wave spectra were obtained from the Swedish Meteorological and Hydrological
Institute (SMHI) and compared against evaluated wave spectra. The hindcast wave spectra represent
analysis fields for the time of concern, computed at European Centre for Medium-Range Weather
Forecasts (ECMWF). In figure 10 and table 2, hindcast and evaluated wave spectra for the position of
M/V Aida at 1200 UTC 2004-02-02 shows fairly good agreement, having almost identical peak
periods. The wave spectra in figure 10 include both wind waves and swell energy. The evaluated wave
spectra indicate however a multiple peak spectra with more wind wave energy (which seems most
reasonable with respect to the wind speed at this time). Table 3 is a corresponding comparison for
2004-02-03 at the time of large rolling. These also compare fairly well, although the evaluated peak
period is approximately 1.5 s larger than for the hindcasts.
As mentioned in the introduction of this report, M/V Aida experienced far more extreme rolling in head
sea last year, in February 2003. As the loading conditions were quite similar, it is interesting to
compare hindcast wave spectra for the two occasions. In figure 10 it can be seen that the hindcast wave
spectra for 2003-02-17, i.e. last years incident, is very similar to the hindcast wave spectra for 2004-02-
02. The peak periods are quite the same and so is the spectral shape, i.e. they are both rather narrow
banded. This strengthens the opinion that last years incident was actually caused by parametric rolling.
It may be noted that the hindcast wave spectra were originally obtained as 3-dimensional spectra. The
3-dimensional spectra were quite centered and symmetric around the main direction. In order to be able
to compare them to the 2-dimensional spectra evaluated by the system, the spectral shape at their main
wave direction was scaled to the same energy content as the original 3-dimensional spectra.

Recordings of head-sea parametric rolling on a PCTC 6(9)
Spectral density (m2s)
14
12
10
8
6
4
Wave spectra comparison
Hindcast 2003-02-17, 1200 UTC
Hindcast 2004-02-02, 1200 UTC
Seaware EnRoute Live 2004-02-02 1200 UTC
Figure 10. Comparison
of wave spectra:
Estimated wave spectra
(Seaware EnRoute Live)
2004-02-02, hindcast
wave spectra 2004-02-
02 and hindcast wave
spectra for last years
incident 2003-02-17.
The peak periods were
almost identical at the
two occasions.
2
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Wave frequency (rad/s)
Table 2. Comparison of significant wave height and peak period 2004-02-02 – hindvast vs evaluated.
Sign. wave height, H s (m) Peak period, T p (s)
Hindcast 5.6 13.4
EnRoute Live ~5.2-6.0 ~12.7-13.2
Table 3. Comparison of significant wave height and peak period 2004-02-03 - hindvast vs evaluated.
Sign. wave height, H s (m) Peak period, T p (s)
Hindcast 12UTC 4.5 12.6
Hindcast 18UTC 4.3 12.6
EnRoute Live 16UTC ~5.1-6.0 ~13.5-14.0
2.5 Measured natural roll periods and calculated encounter periods
As stated earlier, parametrically excited roll is most likely to occur in heading or following waves when
the encounter period is half the natural roll period. The encounter period is defined as:
T = 1
e 1 2πU
cos( β )
, where T is the wave period (s), U is the ship speed (m/s) and β is the
−
T T
2 g
relative wave direction (head waves is here defined as 180 degrees, in opposite to figure 1). Hence the
ship speed and relative wave direction are important factors regarding parametric roll.
The EnRoute system, in automatic mode, continuously calculates the natural roll period (based on
measured roll motion) and the encounter period (based on evaluated wave spectra and ship
speed/heading). In figure 11, some important factors with concern to parametric roll are plotted for a
period around the two events of rolling during 2004-02-03. A slight increase of wave peak period along
with a moderate decrease of speed (probably due to wave impact) matches the events of parametric
roll, “fulfilling the requirements” for parametric rolling to evolve. It is also possible that water on the
forecastle could contribute in triggering the parametric roll by its heeling moment. The master of M/V
Aida suspects that the built-in forecastle could have been filled with water just before the events of
large rolling.

Recordings of head-sea parametric rolling on a PCTC 7(9)
Parametric roll
24
22
20
18
16
14
12
10
Measured natural roll period (s)
Encounter peak period (s)
Wave spectrum peak period (s)
285
280
275
Course (deg)
12
11
10
9
Speed (kts)
8
0 10 20 30 40 50 60 70 80 90
Time (minutes)
Figure 11. Important factors with concern to parametric roll for a period of time during 2004-02-03.
3 Operational guidance regarding parametric rolling
3.1 Implementation of MSC/Circ.707 in Seaware EnRoute Live
Seaware EnRoute Live deals with dynamic stability, such as parametrically excited roll, in route
planning mode, based on weather forecasts, as well as in automatic mode, based on measurements and
evaluated seastate.
The warning functionality in the software regarding dynamic stability is implemented according to
IMO MSC/Circ.707 [1] but provides the possibility to override default threshold values for average
wave length and wave height as well as to extend the formulation to cover all wave directions. The
MSC/Circ.707 guidelines are, in its original form, limited to cover dynamic stability issues in following
and quartering seas. It has for quite some time been known that parametric roll is more likely to occur
in heading waves for certain ship types such as RoRo and large container ships, and that head-sea
parametric roll can in fact be more critical due to strong pitch-roll coupling (leading to large loads on
lashings due to the combined effect of in-phase rolling and pitching). Therefore, the system provides
the possibility to extend the dynamic stability criteria to cover all wave directions. Since it is also well
known that the sensitivity for dynamic stability problems differ a lot between different ships, e.g. [4],
the possibility to modify the threshold values is also provided. This applies in particular to the
significant wave height threshold value. Because of recent head-sea parametric roll incidents with post-
Panamax container ships [2], a revision of MSC/Circ.707 is currently being addressed by IMO, e.g. [3].
The default value for the wave height threshold, according to MSC/Circ707, is 7.6 m for M/V Aida.
However, with facts at hand, it is reasonable to state that a wave height threshold of 7.6 m is too large
for this ship. It has now been reduced to 5 m since we now know it actually occurred in significant
wave heights between 5 and 6 meters.

Recordings of head-sea parametric rolling on a PCTC 8(9)
3.2 Playback of recorded data
During the voyage of concern, the EnRoute system was actively used to adjust the ship speed in order
to avoid bow slamming as far as possible, and the user settings regarding parametric rolling were set
according to MSC/Circ.707, i.e. only following/quartering waves. Consequently, no warnings were
issued since the ship traveled in head sea. It is however possible to use the system in a playback mode
using received weather forecasts and recorded sensor signals, allowing the user to make re-runs with
changed settings. This has been made for the entire voyage using modified settings regarding
parametric rolling, in which all wave directions have been considered and where the wave height
threshold value has been reduced from 7.6 to 5 m.
When the route planning module is re-evaluated using the adjusted dynamic stability settings, the
parametric roll warning is activated for one leg – a leg during which parametric roll occurred 2004-02-
02. In this re-run, a forecast received 2004-01-31 was used - two days ahead of the event. Figure 12 is a
screen dump from this re-run.
Figure 12. Screen dump
from the route planning
module. A parametric
roll warning is raised
for one sub-leg (drawn
in red) during 2004-02-
02, two days ahead of
the event.
Figure 13 presents the result of a playback in automatic mode using adjusted dynamic stability settings,
showing that warnings would have been raised if the modified criteria had been set during the voyage.
Reference time (t=0) is 2004-01-31, 18:34 UTC
20
Parametric rolling
15 Significant roll (deg)
10
5
0
Parametric roll warning active
Advice graph displaying parametric roll warning area
0 10 20 30 40 50 60 70 80 90
Time (hours)
Figure 13. Playback of a part of the voyage. The middle graph shows when the parametric roll
warning was issued (at present condition). The lower graph shows when the advice graph displayed
any warning zones (red area) at any operating condition within the bounds of the advice functionality
(0-23 kts, + - 30 degrees off current course).

Recordings of head-sea parametric rolling on a PCTC 9(9)
3.3 Regarding revision of MSC/Circ.707
The need to extend MSC/Circ.707 to cover also head sea regarding parametric rolling has already been
addressed within IMO, e.g. [3]. The findings of this report provide further grounds for such a revision.
However, the need for further development is also clear. The complexity of dynamic stability
phenomena will probably lead such development towards more rational guidelines or criteria with
focus on operation rather than design. It is a fact that the properties that make, for instance, RoRo and
container ships sensitive to parametric rolling and pure loss of stability are inherent in the actual
concept of those ship types. Attempts to incorporate effects of stability variations in intact stability
regulations will most likely result in higher GM’s, which may not lead to overall improvement due to
increased lateral accelerations. As operational aspects are vital, it is the authors’ opinion that the main
target should be further development of methods for short-term risk assessment that can be put into
practice in ship operation, for instance implemented in operational decision support systems. Such
methods should deal with the specific hull form, taking factors like wave direction, wave spectra
bandwidth and multiple wave systems into account.
Another reflection concerns the awareness of these phenomena among ship officers. By the authors’
experience, very few have heard about parametric rolling, while several seems to recognise the
situation when its nature is described, preferably by showing an animation or model experiment video.
Hence, incorporation of dynamic stability, along with intact and damage stability, in the education of
ship officers may also play an important role.
4 Conclusions
This report presents unique recordings of head sea parametric rolling. It has been shown that the
conditions, in terms of dominant encounter period and natural roll period, matched the primary
parametric resonance very well. The strong pitch-roll coupling during the events of parametric rolling
is also clearly demonstrated.
The recordings of parametric rolling of this report occurred in seastate that hardly can be described as
extreme in any way, having a significant wave height of 5-6 meter and a peak period of 13-14 sec. The
maximum rolling reached 17 degrees and no damages to ship or cargo occurred. When relating these
recordings to the incident of heavy rolling (up to 50 degrees read off inclinometer) in head sea in
February 2003 it is found that the conditions were very similar, except for that the significant wave
height was larger - approximately 7-8 m according to hindcast wave spectra.
It has been possible to adjust the settings, in terms of threshold values and directional concern, in
Seaware EnRoute Live based on the recorded data. When running the system in playback mode with
modified settings, using received weather forecasts and recorded sensor signals from the voyage,
warnings for parametric rolling are issued in route planning mode two days ahead as well as in
automatic (real-time) mode 10 hours prior to the first occurrence.
In order to improve on board guidance, development of rational methods for short-term risk assessment
should continue since the risk for dynamic stability problems can be significantly reduced by weather
routing and proper choice of speed and heading.
5 References
1. Guidance to the master for avoiding dangerous situations in following and quartering seas, IMO
MSC/Circ.707, IMO London, 1995
2. Head-sea parametric rolling and its influence on container lashing systems, SLF 45/6/7, IMO
2002
3. Proposals with regard to the scope of revising the IS code and the related MSC/Circ.707, SLF
45/6/2, IMO 2002
4. Palmquist, M.: On the Statistical Properties of the Metacentric Height of Ships in Following Seas,
Fifth Intl. Conf. On Stability and Ocean Vehicles (STAB 94), Florida, Nov. 1994